i.e., lunar periodicity CKorringa, 1947; Mason, 1958; Mileikovsky, 1970), 

 or various dissolved organic substances (Galtsoff , 1961) . Regardless of 

 what triggers spawning, it is advantageous for the clams to spawn in the 

 fall so their pelagic larvae will have food from the fall phytoplankton 

 bloom. 



Coe and Fitch (1950) reported that Pismo clams in Baja, California 

 (Mexico) have an extended spawning season compared to the clams at San 

 Diego and Pismo Beach; however, length of spawning season does not 

 decrease from San Diego to Monterey Bay. Most gametes are shed in 

 September and October at San Diego (Coe, 1947), Pismo Beach, and Monterey 

 Bay, even though Monterey Bay .is near the northern limit of their range. 

 The average temperature difference between San Diego and Pacific Grove, 

 California, for the spawning months of September and October is 4.82° 

 Celsius (range 2.61 to 6.99), which is based on daily temperatures taken 

 over an 18-year period (U.S. Department of Commerce, 1956). Therefore, 

 Pismo clams spawn at different temperatures at the two locations, 

 suggesting that there is not a single critical spawning temperature for 

 all populations of Pismo clams. However, it does not rule out the 

 possibility that the clams may spawn in response to yearly maximum 

 temperatures occurring in September and October. The spawning season at 

 San Diego may be delayed because spawning is governed by temperature 

 changes over the year. Another possible explanation is that different 

 populations of Pismo clams, as well as the eastern oysters, Crassostrea 

 virginioa (Loosanoff, 1969), are genetically adapted to spawning at 

 different temperatures. 



In Monterey Bay (Fig. 17) and San Diego (Coe, 1947), the Pismo clams 

 first develop mature gonads at about 20 millimeters in length. However, 

 because of a higher growth rate of juvenile clams at San Diego, clams 

 develop mature gonads their first summer in San Diego (Coe, 1947) and 

 their second summer in Monterey Bay. This rapid maturity is .in 

 contrast with other Pacific coast bivalves, such as the smooth 

 Washington clam which matures when it is from 3 to 5 years of age and 

 40 millimeters long, and Nuttall's cockles which matures at 2 years of 

 age anc' 40 millimeters long (Quayle and Bourne, 1972). 



Pismo clams were first thought to be hermaphroditic (Weymouth, 1923), 

 but Coe (1947) later determined the clams were dioecious with less than 

 1 percent hermaphroditic. Samples from Monterey Bay and Pismo Beach 

 showed that less than 1 percent were hermaphroditic. Occasional 

 hermaphrodites are not uncommon in dioecious bivalves (Loosanoff, 1936; 

 Ropes, 1966). The sex ratio of Pismo clams in Monterey Bay was not 

 significantly different from 1:1 in large (more than 44 millimeters) and 

 small (less than 44 millimeters) clams. Coe (1947) found 1.2 females to 

 1 male in large clams in San Diego, which was also not significantly 

 different from a 1:1 ratio. 



43 



